1. Field of the Invention
The present invention relates generally to a method and apparatus for allocating and identifying frequency resources in a Frequency Division Multiple Access (FDMA) system, and in particular, to a method and apparatus for allocating frequency resources of an uplink channel in an Orthogonal Frequency Division Multiple Access (OFDMA) or Single Carrier Frequency Division Multiple Access (SC-FDMA) system.
2. Description of the Related Art
Uplink multiple access schemes recently used in mobile communication systems can be roughly classified into non-orthogonal multiple access scheme and orthogonal multiple access scheme. The non-orthogonal multiple access scheme refers to a multiple access scheme in which uplink signals transmitted from multiple terminals are not orthogonal to each other. Code Division Multiple Access (CDMA) can be a typical non-orthogonal multiple access scheme. The orthogonal multiple access scheme refers to a multiple access scheme in which uplink signals from multiple terminals are orthogonal to each other. FDMA and Time Division Multiple Access (TDMA) can be typical orthogonal multiple access schemes.
In the general mobile packet data communication system, a combined multiple access scheme of FDMA and TDMA is used as an orthogonal multiple access scheme. That is, transmissions of multiple users can be distinguished in the frequency and time domains. In the following description, FDMA refers to a combined multiple access scheme of FDMA and TDMA.
OFDMA and SC-FDMA can be typical FDMA. These FDMA schemes refer to a multiple access scheme in which multiple terminals transmit signals using different sub-carriers so that the terminal signals are distinguishable from each other.
OFDM, a scheme for transmitting data using multiple carriers, is a kind of Multi-Carrier Modulation (MCM) that converts a serial input symbol stream into parallel symbol streams and modulates them with multiple orthogonal sub-carriers, i.e. multiple sub-carrier channels.
FIG. 1 is a block diagram of a general OFDM system transmitter's structure. Referring to FIG. 1, an OFDM transmitter 100 includes an encoder 101, a modulator 102, a serial-to-parallel (S/P) converter 103, an Inverse Fast Fourier Transform (IFFT) processor 104, a parallel-to-serial (P/S) converter 105, and a Cyclic Prefix (CP) inserter 106.
Encoder 101, called a channel encoding block, performs channel coding on an input information bit stream. Generally, encoder 101 can be classified into convolutional encoder, turbo encoder, and Low Density Parity Check (LDPC) encoder, and the like. Modulator 102 performs such modulation as Quadrature Phase Shift Keying (QPSK), 8-ary PSK (8 PSK), 16-ary Quadrature Amplitude Modulation (16 QAM), 64 QAM, 256 QAM, etc. Although not shown in FIG. 1, a rate matching block for performing repetition and puncturing can be added between encoder 101 and modulator 102.
S/P converter 103 serves to convert the output of modulator 102 into a parallel signal. IFFT block 104 performs IFFT calculation on the output of S/P converter 103. The output of IFFT block 104 is converted into a serial signal by P/S converter 105. CP inserter 106 inserts a Cyclic Prefix (CP) into an output signal of P/S converter 105.
IFFT processor 104 transforms frequency-domain input data, into time-domain output data. In the general OFDM system where input data is processed in the frequency domain, if the input data is transformed into time-domain data by IFFT processor 104, the Peak-to-Average Power Ratio (PAPR) increases undesirably. The PAPR can be one of the most important factors that should be considered in uplink transmission. If the PAPR increases, cell coverage decreases, causing an increase in the terminal price. Therefore, in the uplink, there is a need for an attempt to reduce the PAPR. Thus, in OFDM-based uplink transmission, use of an uplink multiple access method modified from the general OFDM scheme can be considered. That is, PAPR can be efficiently reduced with the use of a method of processing (channel coding and modulation) the data in the time domain rather than performing the data processing in the frequency domain.
FIG. 2 is a block diagram of a general SC-FDMA system transmitter. Referring to FIG. 2, an SC-FDMA transmitter 200 includes an encoder 201, a modulator 202, an S/P converter 203, an FFT processor 204, a mapper 205, an IFFT block 206, a P/S converter 207, and a CP inserter 208.
Encoder 201 performs channel coding on an input information bit stream. Modulator 202 performs such modulation as QPSK, 8 PSK, 16 QAM, 64 QAM, 256 QAM, etc. A rate matching block is omitted between encoder 201 and modulator 202. S/P converter 203 serves to convert the output of modulator 202 into a parallel signal. FFT block 204 performs FFT calculation on the output of S/P converter 203.
Mapper 205 maps the output of FFT block 204 to an input of IFFT block 206. IFFT block 206 performs IFFT calculation. The output of IFFT block 206 is converted into a serial signal by P/S converter 207. CP inserter 208 inserts a CP into the output signal of P/S converter 207.
The operation of mapper 205 will now be described in detail with reference to FIG. 3. FIG. 3 illustrates the detailed operation of mapper 205 in FIG. 2.
Referring to FIG. 3, channel-coded/modulated data symbols 301 are input to an FFT block 302, and the output of FFT block 302 is input again to IFFT block 304. Here, mapper 303 serves to map output information of FFT block 302 to input information of IFFT block 304. Mapper 303 maps the frequency-domain signals, which were transformed from the time-domain signals by FFT block 302, to appropriate input points of IFFT block 304 so that they can be carried on appropriate sub-carriers. If the output of FFT block 302 is continuously mapped to the input of IFFT block 304 in the mapping process, consecutive sub-carriers are used in the frequency domain. This multiple access scheme is called Localized Frequency Division Multiple Access (LFDMA).
If mapper 303 maps the output of FFT block 302 to the input of IFFT block 304 while maintaining an equal distance (or regular interval), regular-interval sub-carriers are used in the frequency domain. This multiple access scheme is called Interleaved Frequency Division Multiple Access (IFDMA) or Distributed Frequency Division Multiple Access (DFDMA). In the following description, this multiple access scheme will be referred to as DFDMA, for convenience. FIGS. 2 and 3 show one method of implementing the SC-FDMA technology through frequency axis processing, and several other methods, such as a method of implementing the SC-FDMA technology through time axis processing, can be used.
FIGS. 4A and 4B illustrate positions of DFDMA sub-carriers and LFDMA sub-carriers in the frequency domain, respectively. In a terminal using DFDMA shown in FIG. 4A, sub-carriers are located at regular intervals over the entire frequency domain, and in a terminal using LFDMA shown in FIG. 4B, sub-carriers are consecutively located in a partial frequency domain.
LFDMA and DFDMA have their own unique characteristics. LFDMA, as it uses a partial segment of the entire system frequency band, makes efficient use of frequency scheduling that can select and transmit a partial frequency band having a good channel gain in a frequency selective channel that suffers considerable channel variation in the frequency band. However, DFDMA, as it uses multiple sub-carriers distributed over the broad band, uses frequency diversity gain because it can experience several channel gains together. As described above, in order to maintain the SC-FDMA properties, after passing through only one FFT, the simultaneously transmitted information components should always be mapped to an IFFT block such that they satisfy LFDMA or DFDMA.
In a system using OFDMA or SC-FDMA, when several terminals in one cell have data to transmit over an uplink, the base station needs to allocate frequency resources to the several terminals through scheduling. The scheme of allocating the entire frequency resources to several terminals can be classified into a Localized scheme and a Distributed scheme. When OFDMA uses a Frequency Selective Scheduling scheme or when SC-FDMA uses an LFDMA transmission scheme, the Localized scheme can be used. Otherwise, the base station can allocate frequency resources with the Distributed scheme.
FIG. 5A shows an example of a Localized resource allocation scheme. In FIG. 5A where only the frequency resources used for data transmission are shown, when data is actually transmitted in a physical layer, pilots necessary for the transmission or channels for control information are allocated to other frequency resources and then multiplexed with data channels. The entire frequency band is divided into multiple resource units, and the resource unit is composed of several sub-carriers. The size of the resource unit is determined taking into account overhead of control information during scheduling, gain of scheduling, and a minimum unit of desired transmission data.
In the case shown in FIG. 5A, one resource unit is allocated to a first User Equipment 501 (UE1; or terminal), 3 resource units are allocated to a UE2 503, and 7 resource units are allocated to a UE3 505. In order to inform a corresponding terminal which sub-carrier is allocated thereto, the Localized allocation scheme can simply assign a sequence number to each resource unit and then provide the corresponding number information as channel ID information.
FIG. 5B shows an example of a Distributed resource allocation scheme, which is a DFDMA frequency allocation scheme. Referring to FIG. 5B, a terminal is allocated frequency resources at regular intervals according to the DFDMA characteristics. A UE1 511 is allocated every 12th sub-carrier in the entire frequency resources, and a UE2 513 is allocated every 3rd sub-carrier. In the example where the total number of sub-carriers is 48, UE1 511 is allocated 4 sub-carriers, and UE2 513 is allocated 16 sub-carriers. As a result, the number of sub-carriers available for UE2 513 is 4 times the number of sub-carriers available for UE1 511.
In DFDMA, the regular interval at which sub-carriers are allocated is called Repetition Factor (RF). To acquire information indicating which sub-carrier is allocated to the corresponding terminal, the Distributed allocation scheme can simply acquire the RF and offset information. In other words, RF=12 and Offset=0 for UE1 511, and RF=3 and Offset=1 for UE2 513.
When a base station allocates frequency resources to terminals with the localized scheme, the scheduling information can include Cell Radio Network Temporary Identifier (C-RNTI), Channel ID, and Modulation and Coding Scheme (MCS) information. C-RNTI enables a connected terminal to determine whether an allocated unique terminal ID is assigned to the corresponding terminal. The Channel ID is information used for indicating which frequency resource is allocated to the terminal. The Channel ID indicates information on the resource units in the Localized scheme, and means a combination of RF and Offset in the Distributed scheme.
In order to efficiently schedule frequency resources, the base station needs to schedule the frequency resources taking into account the channel condition or buffer status of terminals every Transmit Time Interval (TTI), which is the minimum transmission unit of data. However, if the C-RNTI information, the Channel ID information, and the MCS information are transmitted to the corresponding terminals every TTI, signaling overhead of the downlink may increase undesirably. Particularly, in Long Term Evolution (LTE), because TTI is very short, i.e. about 0.5 ms, the signaling overhead problem can be more serious.
Therefore, as an alternative to the above method, there are many proposed methods of transmitting scheduling information in order to reduce the signaling overhead. One of them is a scheduling scheme in which for a real-time service like Voice over Internet Protocol (VoIP), once resources are allocated, a terminal occupies the corresponding resources continuously or for a predetermined time until it receives signaling for release. However, in this scheduling scheme, the terminal continuously occupies the allocated resources. Meanwhile, if a cell has surplus resources or runs short of resources, the cell would then need to reduce or add some resources. For this, there is a need for additional signaling information.
In the IEEE 802.20 system, when there is a change in resource allocation, the base station transmits resource allocation information using supplemental bits. This method uses the entire information even when frequency resources are additionally reduced/increased or the completely new frequency resources are allocated, thereby causing an increase in the overhead.